This photo shows Vermont Yankee nuclear power plant as seen from New Hampshire.

A federal court judge has set a hearing date for early June in Entergy Vermont Yankee’s latest legal challenge against the state of Vermont.

The owner of Vermont’s only nuclear plant sued the state last month, charging that state regulators have delayed approval of a backup emergency diesel generator.

Yankee is under orders from the U.S. Nuclear Regulatory Commission to install a backup power source by September. The generator is supposed to be available if the plant is disabled and loses electricity from the transmission grid.

Yankee says it needs to start construction by June 11 to get the equipment online by September. But the Public Service Board has not granted a permit for the project.

Yankee’s lawyers point to an April 24 PSB scheduling order as evidence that the board would not issue a timely decision.

“The order could have, but did not, give assurance that the board would grant the (certificate of public good) before June 11, 2013, or even that the board would issue a final decision … before that date,” Yankee said in its lawsuit.

Vermont Law School professor Pat Parenteau said the suit puts state regulators in a difficult situation.

“The point is you have a federal law that mandates something to be done and the state seems to be frustrating that,” he said. “And that’s just not the position you want to be in.”

Yankee in September asked the board for an expedited review of the generator case. The PSB then opened an investigation into the issue in December. The board, in its April scheduling order, said it is considering whether it can grant approval for the new generator “when Entergy VY is not in compliance with existing (PSB) orders” covering Yankee’s continued operation.

The Vermont Supreme Court is also looking at that question. Entergy has asked the high court to rule that it can continue to operate under the terms of its old certificate of public good, instead of a new one now under review by the PSB.

Parenteau said the latest legal challenge over the generator is part of an all-out effort by Entergy to gain court rulings that federal law pre-empts state control over the plant.

“And so this is just a pattern now of Entergy filing one suit after another forcing that kind of outcome,” he said.

But Parenteau said the practical issue in the generator case is that Entergy is under the gun to install a new power source at Yankee.

“Once again, I’m concerned about how the state interacts with the federal court in this matter,” he said. “If they have a real basis to say, ‘No, we’re not going to let you do this.’ Then, OK, what is that basis? But they don’t seem to have that. Why didn’t the state just do something that would avoid this? I don’t know.”

A spokesman for the PSB said its members would not comment, because the issue is in litigation and involves an open case at the board.

The new lawsuit brings a familiar claim before the federal court. Yankee says once again that federal law trumps state law on issues of nuclear safety.

In 2012, Entergy won a federal pre-emption case in a suit that challenged two Vermont laws that required legislative approval to operate the plant after its state license expired.

In the latest case, the company argues that since the backup generator is needed for safety purposes, Vermont cannot block the project.

The latest court filings include statements from company officials on the need for the generator. The plant has been using a power line from the nearby Vernon dam as an emergency power source. But that alternative will go away in September because of changes to the way the New England electric grid operates in a systemwide blackout. That left a new on-site generator as the only practical option, according to Robert Wanczyk, licensing manager for Entergy Vermont Yankee.

“To my knowledge, no other nuclear power plant operating in the United States has ever been prevented by a state from installing a station blackout generator required by the NRC,” Wanczyk said.

A hearing in the case is set for June 4 at the U.S. District Court in Burlington.